190 Transactions of the Society. 



cerning the comparisons I, amongst others, made of the lower-power 

 apochromatics of Zeiss, further than to remark that in ray judgment too 

 much has been sacrificed to the object of enabling the observer to employ 

 very thick cover-glasses. This is, no doubt, a convenience ; but if, as in 

 Zeiss's 1/4 in. and 1/6 in., the choice lies between object-glasses that 

 cannot be used for covered and uncovered objects, and object-glasses that, 

 with a moderate range of thickness for cover-glass, provide that facility, 

 the latter appear to me, from a practical point of view, to be the 

 better. 



I note with interest that Powell and Lealand have made an achro- 

 matic oil-immersion condenser of N. A. 1 • 4, and will probably be able to 

 increase the aperture to 1*5 in proportion as thinner glass is used to 

 mount objects upon. The mechanical part of this instrument had, 

 when it first reached me, a very neat form, but was diSicult of manipu- 

 lating, and this involving, as it did, alteration, has prevented me from 

 really testing its merits. But I have just received it, with a mechanical 

 modification 1 suggested well carried out, and I have little doubt but I 

 shall realize now its optical excellence. 



On the whole, then, we may rejoice in the fact that a distinct 

 advance has been made in the optics of the Microscope ; and the more so 

 from a conviction that there lies considerable potentiality still in the 

 sources from which the amount of progress made has resulted. 



At the time that I was engaged in preparing to write the Address I 

 had the honour to give to this Society last year, I was for some time in a 

 state of mental indecision as to which of two subjects I should take, the 

 one I selected, or another that had occupied my attention and secured 

 my interest for between sis and seven consecutive years. 



But just a short time before, an accident, which no foresight could 

 have guarded against, happened to the apparatus employed, which 

 occurring in my absence, brought to an abrupt termination the con- 

 secutive observations of nearly seven years. 



This of course greatly depressed me ; for although the observations 

 made were in themselves, and so far as they went, most interesting, they 

 were incomplete ; for the experimental conditions I had set up, and to 

 which I will presently refer, must have ended fatally to the organisms 

 under experiment at some point ; and that point had not been reached. 



As a consequence, I, under the influence of immediate depression, 

 came to the conclusion that I must abandon the whole matter ; and 1 

 gave a brief and rough outline of what I had tried to do, to a local 

 Society and endeavoured to forget it, choosing the subject I had the 

 pleasure of bringing before you last year as my Annual Address. 



But soon I began to look carefully over my records, and to see that 

 what had been done was of real interest ; and I found that going entirely 

 over the ground again, with enlarged knowledge and experience, might 

 after all be a benefit. I therefore restored, renewed, and added to my 

 apparatus, recommenced the observations, and for several months now 

 my thermostat has been successfully at work, repeating the observations 

 of past years. 



I have determined that a record of the former series of observations, 



